let x = [0, 1, null];

let zoo: Animal[] = [new Rhino(), new Elephant(), new Snake()];

window.onmousedown = function(mouseEvent) {
    console.log(mouseEvent.button);  //<- Error
};


window.onmousedown = function(mouseEvent: any) {
    console.log(mouseEvent.button);  //<- Now, no error is given
};


